Glass for optical waveguides or the like

ABSTRACT

Optical waveguides made of quartz glass with reduced infrared absorption and reduced attenuation coefficients are made of glass material composed of atoms having mass numbers higher than that of the natural isotope distribution. The quartz glass or doped quartz glass is made of silicon atoms, of which most or all have the mass numbers 29 and/or 30, as well as of oxygen atoms, of which most or all are composed of isotopes with the mass numbers 17 and/or 18. Atoms of the  76 Ge isotope are preferably used for doping with germanium atoms having higher mass numbers than in the natural isotope mixture. Glass with atoms of preferably  30 Si and/or  18 O are preferably used for optical waveguides based on quartz glass having attenuation coefficients below 0.15 dB/km. As indicated, such optical waveguides are also suitable for transmitting high-energy, pulsed or continuous laser light in a wavelength range from 2.0 to 3.0 μm. These optical waveguides are also suitable for transmitting holmium laser light at 2.1 μm and Er laser light with a wavelength of 2.79 and 2.94 μm.

FIELD OF THE INVENTION

The present invention relates to glass for optical waveguides or thelike.

RELATED TECHNOLOGY

Optical waveguides are being increasingly used as transmission lines intelecommunications networks. In addition, optical waveguides are findingincreasing use in medicine, sensor technology and for the transmissionof high optical powers for the machining of materials. Fortelecommunications, use appears to be made predominantly of single-modefibers made of silica glass with a core that is approximately 10 μm indiameter and a cladding with a lower refractive index and a diameter of125 μm. Some optical waveguides are made of silica glass and arecharacterized by high transmission capacity and low attenuation. At theminimum attenuation given a wavelength of 1.57 μm, values of approx.0.20 dB/km are obtained. In 1986, the reference by G. Tanaka et al.,“Characteristics of Pure Silica Core Single Mode Optical Fiber”,Sumitomo Electric Technical Review 26(1987)43, refers to an optimumvalue of 0.15 dB/km.

The reference Patent Abstracts of Japan: JP-A-60090845, purportedlydescribes a gas rinsing with helium and deuterium during thevitrification of porous SiO₂ (silica glass) and GeO₂-doped silica glassis described. The helium reduces the structure defects in the silicaglass during this process and thereupon completely passes off. Deuteriumis a hydrogen isotope and expels hydrogen atoms from the material, sothat OD-groups instead of OH-groups form in the preform. OH-groups,which can be deemed as impurity in concentrations of less than 0.0001%in silica glass, cause absorption bands in the infrared wavelengthrange. When hydrogen is replaced by deuterium, the absorption bandsshift to longer wavelengths. In this manner, ranges which are important,for example, for optical telecommunications and have additional lossesdue to the OH absorption, are then free from additional absorption. Thistechnology is costly and has not gained acceptance in practice, becausemeanwhile, the fiber manufacturers reduce the OH concentration in thesilica glass to the extent that the OH absorption virtually plays norole any longer.

In addition, in the reference Database, Chemical Abstracts: AccessionNo. 81:44002, a silica glass doped with ²⁹Si is apparently describedgenerally.

The attenuation α of single-mode fibers above a wavelength of 1 μm iscomposed of three components. These are the scattering α_(s), the OHabsorption α_(OH) and the infrared absorption α_(IR). The OH absorptionis due to the inclusion of a small concentration of OH ions in thesilica glass. It is very highly dependent on the wavelength andmanifests itself in the spectral attenuation curve by an absorption bandat approximately 1.4 μm. In the wavelength range of importance foroptical telecommunications between 1.5 and 1.7 μm, the attenuation isvirtually determined only by the scattering losses and the infraredabsorption. The scattering losses are due essentially to Rayleighscattering and decrease with increasing wavelength λ with 1/λ⁴. Theinfrared absorption starts at around 1.5 μm and rises steeply withincreasing wavelength. The minimum attenuation is at 1.57 μm, becausethis is where the decrease of the scattering losses and the increase ofthe infrared absorption are of identical magnitude.

FIG. 1 shows the described situation with reference to the spectralattenuation curve of a single-mode fiber for the wavelength rangebetween 1.1 and 1.7 μm. The OH absorption band at 1.4 μm is clearlyevident. At its minimum at 1.57 μm, this fiber has an attenuation of0.17 dB/km. The characteristic curve of the scattering losses without OHand infrared absorption is shown by the broken line. OH absorption playsvirtually no role for the wavelengths above 1.5 μm. Starting from around1.6 μm, there is then a steep rise in infrared absorption. The referenceby M. E. Lines et al., “Calcium Aluminate Glasses As PotentialUltralow-Loss Optical Materials At 1.5-1.9 μm”, Journal ofNon-Crystalline Solids 107(1989)251, purportedly describes that infraredabsorption is calculated according to the formula α_(IR)=A·e^(−a/1). Inthis context, A=6·10′ dB/km and a=48 μm is given for silica glass. Aninfrared absorption of 0.02 dB/km and 0.33 dB/km is calculated therefromfor 1.55 μm and 1.7 μm, respectively. The infrared absorption in glassesis caused by the tails of extremely strong vibration bands (phonons) inthe long-wave infrared range, as purportedly described by the referenceby S. Kobayashi et al., “Characteristics of Optical Fibers in InfraredWavelength Region”, Review of Electrical Communications Laboratories26,3-4(1978)453. The dominant absorption band of pure silica glass is at9.1 μm and has, at its maximum, an attenuation of 10¹⁰ dB/km. Thespectral position and width of the absorption band are determined by themasses of the atoms involved, i.e., in the case of silica glass, by themasses of silicon and oxygen. The basic physical principles of this arepurportedly presented in detail in the reference by T. Ruf et. al.: “VonFedern und Massen: Physik isotopenreiner Halbleiter” (“Of Springs andMasses the Physics of Isotropically Pure Semiconductors”), PhysikalischeBlätter 52,11(1996)1115. For pure and doped silica glass, which to datehas been employed for the manufacture of optical waveguides, use is madeof natural isotope mixtures of the elements involved, i.e., silicon andoxygen in the case of pure silica glass. Germanium and fluorine aremainly used as dopants.

SUMMARY OF THE INVENTION

An object of the present invention is to provide glass with reducedinfrared absorption, the glass being especially suitable for opticalwaveguides with significantly reduced, minimal attenuation coefficientsand for effective manufacture.

The present invention provides an optical waveguide comprising a glassmaterial including pure or doped silica glass, the pure or doped silicaglass including atoms of at least one element, respective atoms of afirst element of the at least one element having a mass numberdistribution more heavily weighted toward higher mass numbers than anatural mass number distribution of isotopes of the first element.

Owing to the fact that glass is made up of atoms with mass numbershigher than the mass numbers of the natural isotope distribution, theincreased mass numbers (atomic masses) result in a shift of theabsorption bands toward longer wavelengths and in a reduction of theline width. These two effects mean that the start of infrared absorptionis shifted toward longer wavelengths, with the result that there is aconsiderably broader spectrum of application for optical waveguides orthe like made from such glasses. The costs of the optical waveguidesaccount for only a small portion of the total costs, especially in thecase of elaborately manufactured submarine, or undersea, cables. Owingto the reduced attenuation of the optical waveguides according to theinvention, it is thus possible to economize on or entirely dispense withrepeaters/amplifiers, the result being that, in many cases, despiteincreased fiber costs, there are, overall, considerable cost savingswith improved transmission quality in the construction of submarinecable links or for trunk cables over land.

BRIEF DESCRIPTION OF THE DRAWING

The present invention may be more easily understood with reference tothe drawings, in which:

FIG. 1 shows a spectral attenuation curve of a single-mode fiber for thewavelength range between 1.1 and 1.7 μm.

DETAILED DESCRIPTION

First, a table is shown which compiles the stable isotopes of siliconand oxygen according to information from the Physikalisches Taschenbuch(“Physics Pocketbook”), 5th edition, 1976, Viehweg Verlag Braunschweig,published by H. Ebert.

TABLE Oxygen: O Fluorine: F Silicon: Si Germanium: Ge Mass number/ Massnumber/ Mass number/ Mass number/ frequency % frequency % frequency %frequency % 16 99.76 19 100 28 92.2 70 20.5 17 0.037 29 4.7 72 27.4 180.204 30 3.1 73 7.7 74 36.5 76 7.8

The silica glasses for optical waveguides produced until now haveemployed natural isotope mixtures. For pure silica glass, according tothe table of an isotope mixture made essentially of ²⁸Si and ¹⁶O.According to the present invention, the natural isotope mixture or thelight isotopes wholly or partially by heavy isotopes, i.e., to make SiO₂from ³⁰Si and ¹⁶O, or from ³⁰Si and ¹⁸O, or from ²⁸Si and ¹⁸O, arereplaced. The sum of the increased mass numbers (atomic masses) causes ashift of the absorption bands toward longer wavelengths and a reductionof the line width. These two effects mean that the start of infraredabsorption is shifted toward longer wavelengths. According to thereference by T. Ruf et al., “Von Federn und Massen: Physikisotopenreiner Halbleiter”, Physikalische Blätter 52,11(1996)1115, theshift of the spectral position of the absorption bands is proportionalto 1/{square root over ( )}M (M is the mass number) and the width of theabsorption band is proportional to 1/M. If ²⁸Si is replaced by ³⁰Si,this results, for the band at 9.1 μm, in a shift to 9.25 μm, whichcorresponds to a difference of 0.15 μm. The reduction of the width ofthe absorption band basically has twice the effect on the start ofinfrared absorption in the shortwave spectral range, thus also in theattenuation minimum, with the consequence that, for an optical waveguidemade of ³⁰Si and ¹⁶O, appreciable infrared absorption starts only at1.95 μm instead of at 1.5 μm. Therefore, the attenuation minimum isshifted to 1.95 μm. With the hitherto achieved values for scatteringlosses in silica glass optical waveguides, there is then an attenuationcoefficient of less than 0.1 dB/km.

If a silica glass is made from ³⁰Si ¹⁸O, the start of infraredabsorption is shifted by approximately 1.35 μm, i.e., the attenuationminimum is then shifted to 2.85 μm, this resulting in an attenuationcoefficient of less than 0.05 dB/km.

Optical waveguides with attenuation coefficients less than 0.1 or 0.05dB/km open up new possibilities for submarine, or undersea, cables, inparticular. Namely, until now, it has been necessary to equip submarinecables with repeaters or optical amplifiers at specified distances,which necessitate remote electrical power feeding for the energy supply,thereby resulting in high costs. Repeater-/amplifier-free cables havebeen developed for the offshore region, such cables being capable ofspanning a maximum of 500 km with a fiber attenuation of 0.2 dB/km. Ifoptical waveguides with less than 0.1 dB/km are made available by theapplication of the glass according to the invention, it will now bepossible for all continents to be linked by such submarine cableswithout the use of amplifiers or repeaters.

Although high costs are involved in manufacturing glasses from pureisotopes or from isotope mixtures whose distribution differs from thenatural distribution, these costs do not play any decisive role in thepractical application of the proposed embodiment for the followingreasons.

The optical waveguides or their preforms can be produced according to aso-called casing process which is already in practical use today. Inthis process, only the inner preform, i.e., the core and a region nearto the core, is manufactured according to the conventional process.Melted onto this inner preform is a thick-walled tube of pure silicaglass (casing tube) whose production costs, based on the identicalquantity, are considerably lower than those for the inner preform.

The material constituent of the inner preform can be reduced toapproximately 5% of the total volume without there being any increase inattenuation with the fibers manufactured from such preforms. The resultof this is that only 5% of the fiber material must be produced fromglass of modified isotope composition. For one kilometer of single-modefiber, this is equivalent to a mass of less than 2 grams.

For elaborately manufactured submarine cables, the costs for the opticalwaveguides account for only a small portion of the total costs. If, dueto lower attenuation of the optical waveguides, it becomes possible toeconomize on or to dispense entirely with repeaters and amplifiers,despite increased fiber costs, the overall result will be considerablylower costs with improved transmission quality and reduced frequency ofrepair for applications in long submarine cable links or even for trunkcables over land.

Only minor changes in infrared absorption are to be expected from thereplacement of the natural isotope distribution of the dopants byheavier isotopes. There is only one stable isotope in the case offluorine. The mass number of all germanium isotopes is well above theatomic mass number of silicon and oxygen. Furthermore, in standardsingle-mode fibers, the type of fiber most frequently used intelecommunications, the core material is doped only with about 5% GeO₂.Conversely, in the case of higher doping rates, such as indispersion-shifted single-mode fibers, it is possible to achieve a smallshift of the infrared absorption through the use of ⁷⁶Ge in relation toglass which is doped with a natural Ge isotope mixture.

As already mentioned, telecommunications is not the only area ofapplication for silica glass fibers made of heavy isotopes with infraredabsorption shifted into the longwave spectral range; further importantapplication areas also exist, for example, in industrial and medicalapplications with infrared lasers.

Light absorption, for example in tissue or in blood, is particularlyhigh both in the UV range, caused by protein and water absorption, andalso in the infrared range, caused by water absorption. The penetrationdepth for UV laser light and also for the light of a CO₂ laser (10.6 μm)and of the Er:YAG (at 2.94 μm) and Er:YSGG (at 2.79 μm) is in the rangebetween 1 and 20 μm. Owing to this short distance in which the absorbedenergy is converted into other forms, Er lasers, above all, are ofinterest as highly efficient and reliable sources of radiation formedical applications. Such medical applications require opticalwaveguides approximately 2 meters long whose overall transmission shouldnot be considerably below 50%, i.e., the required attenuation is of theorder of magnitude of max. 3 dB.

It is also known that, of optical fibers made of a variety of materials,such as chalogenides, halogenides, sapphire, waveguides, fluid lightguides, none approaches the qualities of silica glass fibers. However, amajor disadvantage of standard silica glass is that the attenuation atEr laser wavelengths is so high that applications have hitherto beenlimited to very short sections of fiber.

It has now been established that a silica glass fiber at 2.94 μm has anattenuation factor of 12 dB/m, i.e., after 2 meters fiber length, lessthan 1% of the coupled-in optical power emerges at the end of the fiber.On the other hand, if a silica glass made of ³⁰Si¹⁶O₂ is used, theabsorption edge shifts by approximately 0.3 μm in this range, which,with regard to the logarithmically represented attenuation, lies in themiddle between the absorption minimum and maximum. The infraredabsorption at 2.94 μm is thereby reduced to approximately 1.8 dB/m. Itmay be that this value is considerably higher than the previouslydescribed minimal attenuation factor, however, it is entirely sufficientfor the applications in this wavelength range. Approximately 40% of thecoupled-in radiant power now still exits at the end of a 2-meter-longglass fiber. This transmission value lies in the aforementioned order ofmagnitude of about 50%. On the other hand, the other advantageouscharacteristics of silica glass and silica glass fibers are not changed.This makes it possible to implement new silica glass fiber transmissionsystems in medical technology at 2.94 μm, this being accomplished inconjunction with cost-effective Er lasers.

Furthermore, owing to the lower absorption, the transmitted powerdensity or power can be considerably increased, and specifically tovalues which would result in overheating and destruction with thehitherto known fibers. This, therefore, also opens up new areas ofapplication in industry, especially in the machining of materials.

Even for these additional applications, the material usage of (heavy)silicon dioxide (³⁰Si¹⁶O₂) is low. Although the fibers are considerablythicker, and specifically with typical core diameters of 200 to 600 μmand a typical cladding-core ratio of 1.1 to 1.4, only isotope materialin the gram range is required owing to the considerably shorter fiberlength of approximately 2 meters. A fiber with a core diameter of 200 μmand a cladding diameter of 240 μm weighs only approximately 0.2 grams.

What is claimed is:
 1. An optical waveguide comprising a glass materialincluding pure or doped silica glass, the pure or doped silica glassincluding atoms of at least one element, respective atoms of a firstelement of the at least one element having a mass number distributionmore heavily weighted toward higher mass numbers than a natural massnumber distribution of isotopes of the first element.
 2. The opticalwaveguide as recited in claim 1 wherein the first element is silicon,the respective atoms of the silicon being predominantly or entirelyisotopes having respective mass numbers of 29 and/or
 30. 3. The opticalwaveguide as recited in claim 1 wherein the first element is oxygen, therespective atoms of the oxygen being predominantly or entirely isotopeshaving respective mass numbers of 17 and/or
 18. 4. The optical waveguideas recited in claim 1 wherein the first element is silicon and a secondelement of the at least one element is oxygen, the respective atoms ofthe silicon being predominantly or entirely isotopes having respectivemass numbers of 29 and/or 30, respective atoms of the oxygen beingpredominantly or entirely isotopes having respective mass numbers of 17and/or
 18. 5. The optical waveguide as recited in claim 1 wherein thematerial is doped silica glass, the doped silica glass being doped withatoms of isotope ⁷⁶Ge.
 6. The optical waveguide as recited in claim 1wherein the at least one element includes at least one of the firstelement, oxygen and germnanium, the first element being silicon,respective atoms of the oxygen having a mass number distribution moreheavily weighted toward higher mass numbers than a natural mass numberdistribution of oxygen isotopes, respective atoms of the germaniumhaving a mass number distribution more heavily weighted toward highermass numbers than a natural mass number distribution of germaniumisotopes.
 7. The optical waveguide as recited in claim 6 wherein thesilicon includes ³⁰Si, the oxygen includes ¹⁸O, and the germaniumincludes ⁷⁶Ge.
 8. The optical waveguide as recited in claim 1 whereinthe pure or doped silica glass has an attenuation of less than 0.15dB/km.
 9. The optical waveguide as recited in claim 1 wherein theoptical waveguide includes a conical profile in a coupling-in region soas to reduce a power density in the coupling-in region.
 10. The opticalwaveguide as recited in claim 1 wherein the optical waveguide isadaptable to transmit at least one of high-energy laser light in awavelength range from 2.0 to 3.0 μm, pulsed laser light in a wavelengthrange from 2.0 to 3.0 μm and continuous laser light in a wavelengthrange from 2.0 to 3.0 μm.
 11. The optical waveguide as recited in claim9 wherein the optical waveguide is adaptable for transmitting an erbiumlaser light including wavelengths of approximately 2.79 and 2.94 μm. 12.The optical waveguide as recited in claim 1 wherein the opticalwaveguide is adaptable for use in at least one of undersea cables,undersea trunk lines, short-distance waveguides, medical technology andmachining of materials.
 13. The optical waveguide as recited in claim 1wherein the optical waveguide includes doped silica glass, the dopedsilica glass including a doping having an isotope distribution differentfrom a natural isotope distribution so that the optical waveguide isadaptable for energy transmission using infrared radiation.
 14. Theoptical waveguide as recited in claim 1 wherein the optical waveguide isadaptable for transmitting a holmium laser light including a wavelengthof 2.1 μm.